1. Field of the Invention
The present invention relates to an optical information recording medium configured to be capable of recording and reproducing record data by irradiating a laser beam to a recording layer formed on a substrate.
2. Description of the Related Art
As an optical information recording medium of this kind, a write-once optical information recording medium has been conventionally proposed which includes a recording layer formed of an organic dye-based material and has the optical characteristics of the recording layer changed as a laser beam is irradiated thereon. Meanwhile, in recent years, optical information recording media are desired to have the capability of recording and reproducing a large amount of record data at a high speed (in a short time period). Accordingly, the optical information recording medium of the above-mentioned kind is required to enhance recording density of record data, and to meet the requirement, the diameter of a beam spot of a laser beam used for recording or reproduction of record data tends to be reduced. More specifically, record data are recorded and reproduced using a pickup which is equipped with an objective lens having a numerical aperture (NA) of not less than 0.7 (e.g. a numerical aperture (NA) of approximately 0.85), and is capable of emitting a laser beam having a wavelength (λ) of not more than 450 nm (e.g. a wavelength (λ) of approximately 405 nm). However, when a recording layer is formed by an organic dye-based material, it is difficult to change the optical characteristics of the recording layer using a laser beam with a wavelength of 405 nm or so. Therefore, the optical information recording medium including a recording layer formed of an organic dye-based material suffers from the problem that it has difficulty in performing high-density recording for enabling a large amount of record data to be recorded and reproduced at a high speed.
On the other hand, an optical disk is disclosed in Japanese Laid-Open Patent Publication No. S62-204442, which includes a recording layer capable of recording record data thereon by a phase change to a crystalline state or an amorphous state which is caused by irradiation of a laser beam thereon. This optical disk is comprised of a protective film, a recording layer, a protective film, and a protective film, sequentially deposited on a substrate in the mentioned order. In this case, the protective films deposited in a manner sandwiching the recording layer are provided for preventing the recording layer from flying off or being bored during irradiation of the laser beam (during recording of record data), and made of SiO, SiO2, SiN3, or the like. Further, the recording layer is formed by depositing two recording films formed of respective materials different from each other. More specifically, for example, one of the recording films is made of Si or Te, and the other is made of Au, Ag, or Ge.
For example, when the one of the recording films is made of Si, and the other is made of Au, both of the recording films are mixed with each other by being irradiated with the laser beam, whereby a single layer of AuSi is formed. In this case, to form the recording layer such that it is changed to a crystalline state or an amorphous state according to the irradiation power and irradiation time period of the laser beam, it is required that the atomic percentage of Si to the whole single layer of AuSi is within a range of 20 at % to 30 at %. Therefore, in the proposed optical disk, the recording films are formed by defining the ratio between the thickness of the one recording film and that of the other to be within a range of 2:8 to 3:7, whereby the atomic percentage of Si to the whole single layer of AuSi is caused to be within the above range. The uppermost protective film, which is the uppermost layer, prevents the other two protective films and the recording layer from being scratched, and is in the form of a thin film made of an ultraviolet-curing resin such that it covers the protective film immediately thereunder. Differently from an optical information recording medium having a recording film formed of an organic dye-based material, the above optical disk is capable of recording and reproducing record data using a laser beam having a relatively short wavelength. Therefore, with this optical disk, it is possible to perform high-density recording using a laser beam which is small in the diameter of a beam spot thereof.
However, as a result of the study of the above prior art, the present inventors have found the following problems: In the conventional optical disk, the recording layer is selectively changed to a crystalline state or an amorphous state by adjusting the irradiation power and irradiation time period of the laser beam, whereby the optical characteristics of the recording layer are changed. In this case, if the optical disk whose recording layer has been changed into a single layer of AuSi by being irradiated with the laser beam (i.e. which has record data recorded on the recording layer) is left standing for a long time period, the level of noise in a reproduction signal is progressively increased to decrease the C/N ratio of the reproduction signal, whereby it becomes difficult to normally reproduce the recorded data. As described above, the conventional optical disk suffers from the problems of a large degree of deterioration caused by aging with respect to the noise level (hereinafter also referred to as “degree of noise-causing deterioration”), which makes it difficult for the conventional optical disk to store record data for a long time period such that the record data can be normally reproduced during the long time period.